You're kind of vague on what you mean by ‘successful’ - do you mean in a popularity sense or in a “Now that's what webcomics should BE!” sense?The BIG webcomics are often the ones that were there at the beginning - Penny Arcade being the 800lb Gorilla (to my knowledge the only webcomic that has had a series of computer games made of it). Harking back to those times, popular webcomics are often a bit geeky, for example XKCD, 8-Bit Theatre and Order of the Stick (all of which also demonstrate that you don't need great art to be popular).My personal list of 10 BEST comics is always changing. There are comics I used to read religiously that I can't even remember now. Off the top of my head I would probably say my top 5 is something like:

AcrobatAdventures of Dr McNinjaBad Guy HighGoblinsSHELL

(Yes, there's a superhero bias there). These are all comics that have been executed well and I've really enjoyed.